Cable pulling machine

ABSTRACT

A cable pulling machine of the invention includes a frame, a mechanical linkage including first and second links having first and second cable gripping shoes disposed thereon respectively in opposed positions, a latching cylinder actuable to cause the mechanical linkage to pivot on multiple axes to bring the shoes into engagement with a cable disposed therebetween, a main cylinder having one end mounted on the frame and actuable to move the linkage in a manner effective to pull the cable in a pulling stroke once the latching cylinder has brought the shoes into engagement with the cable, and a mechanism for extending and retracting the main and latching cylinders. The machine may also include a mechanism for holding the cable in tension between pulling strokes, such as a pivoting wedge mechanism as described hereafter.

FIELD OF THE INVENTION

The invention relates to cable pulling machines of the type used forpulling an underground mole through a pipeline in order to burst thepipeline.

BACKGROUND OF THE INVENTION

Methods and devices to replace existing sanitary sewer pipes by pipebursting are well known in the water and sewer industry. Thesepipe-bursting methods have been in use for at least 15 years. Laterals,those short lengths of pipes that connect a commercial or residentialbuilding to the ‘main’ or collector pipe in the adjacent right-of-way,can be replaced using pipe bursting. Replacement of laterals usingpipe-bursting is becoming nearly as common as replacement of the main.

There are many reasons the pipe that makes up the lateral might needreplacement. Most often the cause is tree roots that grow into thejoints between pipe sections. Over time, as these roots grow thick fromtheir successful search for water, the pipes will begin to crack. Thegaps will widen and permit an inflow of ground water into the sanitarysystem. The first clue that these roots are in the sewer lateral may bein the form of a backup. The homeowner or landlord, who is often legallyresponsible for the condition of the pipe traversing the property, maycall a plumber who can remove the roots, by a process often referred toas ‘roto-rooting’.

While the roots may be removed from the interior of the pipe by thisprocess, and blockage may not occur again for months or years, theexistence of the damaged pipe creates a bane for the local sewerdistrict. Upon entering a period of rainfall, the water table will riseand ground water, usually referred to as clear water, will enter intothe sewer through the leaks caused by the roots. This water, now mixedwith the sanitary flow must be treated as sewage. Unfortunately,rainfall may cover the entire sewer district, and the existence ofleaking laterals is more common than not in older areas. For thisreason, many municipalities experience sanitary flow beyond theircapability to process when even moderate rainfall is experienced. Forexample, Milwaukee, Wis. regularly discharges untreated sewage into LakeMichigan. Further, the District of Columbia has an average of (70)discharges of untreated raw sewage into the Anacostia River each year.The source of these increased flow rates is always at least partiallydue to leaking pipes allowing ground water to enter the system.

The device and method of the invention are an improvement on a basicconcept used in pipe bursting. A cyclic winch applies tension to a wirerope. The rope is further attached to a pipe-bursting device. The deviceor “mole” may be a simple conical shape, an air-actuated impact tool, ahydraulically actuated device that expands radially upon demand, or whenslitting steel, it may be a bladed slitter using can-opener like bladesto separate the pipe and allow subsequent expansion. The known processcalls for the simultaneous installation of High-Density Polyethylenepipe (hereafter HDPE) or similar product pipe. The product pipe is towedbehind the pipe-bursting device.

While cyclic winch devices of this type currently exist, and haveexisted for more than a decade in the pipe bursting field, such useeither completely hydraulically induced grip forces, or in some designs,the grip is entirely self-actuating. In the case of the completelyhydraulically induced grip device, the magnitude of the grip on the ropeis independent of the pulling force. For this reason, the grip forceapplied on every cycle must be capable of keeping the rope from slippingshould maximum rope tension be needed. This full grip force is appliedwhether the current job conditions require it or not.

As the grip force applied to the rope perpendicular to the rope's axisis generally 4 to 6 times the tension force, each gripping cycle cancause fatigue damage to the rope's individual wires.

A seemingly improved alternative is to use self-actuating clamp. In thiscase, friction-inducing teeth, which are actually grooves with sharpedges located on the clamping surface, will start the gripping process.This self-actuating device, which may be either a pair conical collets,wedge shaped, or a four bar linkage, creates a gripping force that isproportional to the axial rope tension. With such a device, the gripforce, while still 4 to 6 times the ropes axial tension, need only gohigh enough to keep the grip from slipping. These devices have twoweaknesses. During initiation, if dirt, grease or oil is present on thecollet or rope surface, there may not be enough friction to induce orinitiate the self-gripping process. Dirt, grease and other slick matterthat may cause the rope not to initiate a grip are all substances thatexist at the bottom of a sewer pipe. It should be assumed they will coatthe wire rope and likely affect the dependability of the grip initiationprocess.

Further, the sharp edged grooves or teeth that help to initiate the gripwhen the cable tension is low, will damage the rope when loads are high.When the axial pull force is very high, the similarly high grip forcemay tend to cut the rope at locations where the collets have sharpfeatures to initiate the process. Thus, what initially seemed like agood technical alternative, has functional weaknesses. The presentinvention addresses these weaknesses.

SUMMARY OF THE INVENTION

A cable pulling machine of the invention includes a frame, a mechanicallinkage including first and second links having first and second cablegripping shoes disposed thereon respectively in opposed positions, alatching cylinder actuable to cause the mechanical linkage to pivot onmultiple axes to bring the shoes into engagement with a cable disposedtherebetween, a main cylinder having one end mounted on the frame andactuable to move the linkage in a manner effective to pull the cable ina pulling stroke once the latching cylinder has brought the shoes intoengagement with the cable, and suitable means for extending andretracting the main and latching cylinders. The machine may also includesuitable means for holding the cable in tension between pulling strokes,such as a pivoting wedge mechanism as described hereafter.

In a preferred form of the invention, the first link is secured to theframe by a first pivot at one end portion of the first link, and has thefirst shoe mounted thereon at a position offset from the first pivot.The first shoe has a first gripping surface configured for gripping acable. The second link is secured to one end of the main cylinder, andthe second shoe has a second gripping surface configured for grippingthe cable, which second gripping surface opposes the first grippingsurface. The latching cylinder is mounted at opposite ends thereof tothe first and second links at positions effective for causing the firstand second gripping surfaces to grip a cable disposed therebetween. Thesecond link may be connected by a second pivot to an end of the maincylinder opposite the main cylinder end connected to the frame. Thesecond link has the second shoe mounted thereon at a position offsetfrom the second pivot. The latching cylinder is preferably pivotallymounted by third and fourth pivots mounted in holes in the first andsecond links respectively. At least one of the shoes may be pivotallymounted to its associated link by a shoe pivot, on which the shoe pivotsas it tightens on the cable. The first and second links overlap oneanother in a lateral direction transverse to the pulling stroke, and areinterconnected for relative rotation by a fifth pivot, the fifth pivotbeing located between the first and second pivots at a positioneffective to cause the links to bring the shoes together to grip a cableupon actuation of the latching cylinder. The lines of action of thelinks, defined hereafter, preferably start in a parallel ornear-parallel condition and move to a beyond parallel condition definingan acute angle preferably ranging from about 5° to 15°. Such a linkageis configured to mechanically magnify gripping force applied to thecable by the latching cylinder, which latching cylinder exerts lessforce than the main cylinder, while at the same time not furtherincreasing the gripping force once the shoes have engaged the cable andthe shoes have moved beyond the parallel position.

According to a further aspect of the invention, a control unit isprovided having a circuit valve which, upon actuation for a pullingstroke, activates the latching cylinder until a predetermined pressureis reached, and then activates the main cylinder. A manual control isprovided to cycle the machine to perform a pulling stroke and then resetto a starting position to repeat for a successive pulling stroke.

A method for the trenchless replacement of an underground pipe using acable pulling machine such as the foregoing includes the steps of:

disposing a pulling cable through the pipe with a pipe bursting moleattached to one end of the cable so that the mole is in position toburst the pipe starting at one end of the pipe;

positioning a cable pulling machine at the other end of the pipe in aposition for pulling the mole therethrough; and

pulling the mole through the pipe using the cable pulling device bysuccessively actuating a latching cylinder of the cable pulling machineso that a pair of cable gripping shoes engage the cable, then actuatinga main cylinder of the cable pulling machine to accomplish a pull on thecable, then releasing the cable by reverse actuation of the main andlatching cylinders, then repositioning the pulling machine to engage afurther length of the cable with the cable gripping shoes to complete apulling cycle, and then repeating the pulling cycle in a series ofpulling strokes.

Other aspects of the invention are set forth in the detailed descriptionthat follows. Among these are a cable pulling system that includes ahydraulic cable pulling machine, and a control stand including means forcontrolling flow of hydraulic fluid to the machine in combination withwheels and a platform suitable for lifting and transporting the machinethereon in the manner of a dolly. The machine may be the one describedherein or one of a completely different design. Hoses are used toconnect the control stand to the machine for conducting hydraulic fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, wherein like numerals denote likeelements:

FIG. 1 is a front view of a wire rope pulling device of the invention,with the main extension cylinder is shown in the retracted position;

FIG. 2 is a right side view of the device of FIG. 1;

FIG. 3 is a back view of the device of FIG. 1;

FIG. 4 is a left side view of the device of FIG. 1;

FIG. 5 is a bottom view of the device of FIG. 1;

FIG. 6 is a top view of the device of FIG. 1;

FIG. 7 is a perspective view of the device of FIG. 1;

FIG. 8 is a sectional view taken along the line 8–8 in FIG. 1, with thedevice shown at the start of a pull;

FIG. 9 is a sectional view taken along the line 8–8 in FIG. 1, with thedevice shown grabbing the wire rope;

FIG. 10 is a sectional view taken along the line 8–8 in FIG. 1, with thedevice shown pulling the wire rope;

FIG. 11 is a sectional view taken along the line 8–8 in FIG. 1, with thedevice shown releasing the wire rope;

FIG. 12 shows the pulling device of FIGS. 1–11 connected to a controlstand according to the invention; and

FIG. 13 is a side view showing a pulling device of the invention inposition for transport on the control stand.

DETAILED DESCRIPTION

According to the invention, a latching cylinder is used to initiategripping of the cable. The gripping mechanism is designed togeometrically increase the cable clamping force as the pulling forceincreases, preferably without the use of teeth on the cable pullingjaws. The puller of the invention uses a gripping mechanism unlike anycollet. Additionally, unlike other four bar linkage designs, the deviceis initiated, or helped to start the grip, with a small hydrauliccylinder. As the axial wire rope load increases as the machine continuesits cycle, the effect of the small hydraulic cylinder becomesunimportant. The mechanism builds gripping force in proportion to themagnitude of the axial pull force. By using the method described, thedesign overcomes the weakest aspect of collets, and also avoids the ropecrushing nature of a simple hydraulic clamp. It is an evolutionarydesign change that makes the machine more dependable and useful thaneither a collet machine, or a full hydraulically induced grip design, ora non-hydraulically initiated four bar linkage.

In use, the cable pulling machine will normally be located in anexcavation placed adjacent to either the connection to the main, oradjacent to the building structure. The machine, using 5 to 30 tons ofaxial wire rope tension, draws the pipe bursting device through theexisting pipe, it splits or bursts the pipe and simultaneously expandsthe ground and installs the product pipe. Upon entering the exit pit,the pipe-bursting device is generally disconnected from the cable andthe cable disengaged from the puller. These components are removed fromthe excavation, the pipe connections are completed and the pit will befilled in and the surface restored. To replace a typical 75 foot longlateral with 4″ HDPE pipe, the entry and exit excavations may be 3′wide×4′ long. Smaller excavations are possible, however that small sizerestricts the ability of the man to perform the pipe connections afterthe pipes have been burst or split.

The device of the invention acts by applying tension to a wire rope andpulling that rope through a distance. The following description willdetail the cyclic nature of that process and other related functions.

Referring to FIGS. 1–9, when an operator shifts the handle 37 for ahydraulic spool valve 45, pressurized hydraulic oil flows to asequencing valve 30. The sequencing valve 30 functions to direct flow totwo separate components in chronological order. That is, flow will go tothe first circuit until a predetermined pressure is reached within thevalve body. Upon attaining that pressure, the second circuit will thenreceive oil, as explained hereafter.

A pulling device or machine 33 of the invention includes a firsthydraulic cylinder 1, known as the latching cylinder because it acts asan over-center latch for the gripping mechanism. This double-actingcylinder 1 is designated as the first circuit in the hydraulic system.Ported with an inlet 19 to receive oil on the rod side, actuation ofcylinder 1 will cause the rod to retract. Retraction of cylinder 1causes rotation of a first link 10 about a pin joint 11 and about a pin13 mounted on a second floating link 8 (FIGS. 8–9). Pin 11 is mounted ona frame 48 of the pulling machine and does not otherwise move relativeto the linkage to be described. One end of cylinder 1 is pivotallyconnected by a pin 53 to an upward extension 52 of link 8. Accordingly,retraction of cylinder 1 causes link 10 to pivot clockwise about pin 11,rotating link 10 downwardly to the position shown in FIG. 9. Pin 13moves downwardly and transmits such movement to link 8 to which it isconnected.

Link 8 rotates about pin joints 9 and 13, and a second cylinder 16 willrotate slightly about pins 12 and 9 at its opposite ends. Pivots 11 and12 are rotatably mounted by brackets on an end wall 59 and base plate 62of frame 48, and brace the linkage so that forces generated duringpulling are transmitted to the frame which has been braced againstsuitable surfaces, such as the side and floor of an excavation.

A first gripping shoe 3 is rigidly attached to link 10 at the end remotefrom pin 11 and will follow an arcuate path about pin 11. The transitionbetween the positions of FIGS. 8 and 9 is the result of these movements.Cylinder 1 is mounted by pivot pins 51, 53 at either end to links 8, 10respectively and pulls the upper portions of links 8, 10 closer togetheras it contracts.

As cylinder 1 completes its stroke, a second shoe 4 makes a slightangular adjustment about a pin 14 by which it is mounted on link 8 atthe end remote from pin 9. This angular adjustment is the product ofmating to a cable or wire rope 31 in a clamping manner. Shoes 3, 4 haveopposed, semi-cylindrical gripping surfaces 17, 18 configured to grip acable of the desired diameter. Surfaces 17, 18 may be plain or knurled,but preferably lack teeth or other sharp projections that would tend tocut the cable. Shoes 3, 4 are aligned to keep the wire rope 31 trappedbetween the shoes during device operation. The profile of rope 31 willbe centered within the lateral confines of shoes 3, 4.

Motion of links 8, 10 as described above brings the gripping contours orsurfaces 17, 18 closer. This serves to clamp or grip the wire rope 31with great force between the shoes 3, 4. This clamp load createsfriction between wire rope 31 and shoes 3, 4. That friction is needed toapply the pulling force to the wire rope 31. The geometric configurationof the pins and links causes the relatively moderate force applied byextension cylinder 16 to be multiplied at shoe gripping surfaces 17 and18. Linkages with multiplying lever effects can be designed in a varietyof ways, of which the one presented herein is only one example. Forbetter stability, link 8 preferably comprises a pair of side plates 90,91 disposed on either side of link 8 and secured together by weldedcrossbars 92. Pins 9, 13, 14, and 53 are mounted in pairs of alignedholes in plates 90, 91 as shown. Plate 91 has an integral reinforcingbar 96 and bosses 97 on its outward side configured for mounting thepins 13 and 14 therein, as shown in FIG. 4, and similar structures areprovided on the opposite side (FIG. 2).

A unique and important aspect of the geometry of links 8, 10 and shoes3, 4 will be described in greater detail to gain a full understanding ofthe invention. By drawing a line from pin 13 to pin 14, the line ofaction of link 10 and shoe 3 is represented. In the illustratedembodiment, shoe 3 cannot pivot relative to link 10, but such a pivotingfunction could be provided. The line of action of link 8 and shoe 4 isrepresented by a line drawn from the center of pivot pin 14perpendicular to gripping surface 18 and wire rope 31 held therein. InFIG. 8, these lines are approximately parallel. The motion of retractingcylinder 1 causes these lines of action to move in relative directionsthat take them away from the parallel condition, as demonstrated inFIGS. 9 and 10. Normally, links are made to be parallel to induce largeforces. An example of this is the commonly-studied Vise Grip_(tm) stylepliers. In that case, a link length is adjusted to cause the componentsto minutely exceed the parallel link condition. Nearly parallel linksfunction to multiply forces greatly as they act much as infinite ratiolevers.

As shown in FIG. 8, in a starting position, the distance between shoecontours 17 and 18 is at its greatest. As the cylinders 1 and then 16are actuated, the links 8, 10 move beyond parallel (past center) suchthat an angle A which is an acute angle defined by the lines of actionreferred to above changes from 0 degrees to a value greater than 0degrees, preferably in the range from 5 to about 15 degrees, 12° in theexample shown. To make this geometry change a benefit, shoes 3 and 4 areconfigured about pivots 13 and 14 to cause gripping surfaces 17 and 18to become closer together as links 10 and 8 move angularly away (i.e.,as angle A increases.) As link 8 rotates about pin 13 in a directioninduced by retraction of latching cylinder 1, shoe 4 orbits (moves alongan arc) about pin 13. The contour of gripping surface 17 on shoe 3 isnominally aligned with the wire rope 31 extending around sheave 5. Whenwire rope 31 is present and wrapped about sheave 5, angular movement oflink 8 and shoe 4 in the direction previously described causes grippingsurfaces 17 and 18 to become closer together and wire rope 31 wrapsaround extension 20 as shown in FIG. 10. This mechanism yields anadvantage of operating successfully within a limited range of cablediameters without the need to replace shoes 3, 4 with shoes havingdifferently sized gripping surfaces 17, 18.

Continued chronological description of the operation takes up with thesequencing valve 30 experiencing the predetermined hydraulic pressurethreshold. Main cylinder 16 then receives oil flow to inlet port 15.This flow, instigated by the logic of the sequencing valve 30, serves toextend cylinder 16 and thereby induce further linkage displacement ofall pivoted members previously described. This displacement creates yetmore clamp load between shoes 3 and 4. Additional clamp load, over andabove that produced by the latching cylinder 1, is required to achievethe desired pulling forces. The linear movement of the wire rope 31 andtherefore the useful progress of the pipe bursting process ismathematically related to, but not of the same magnitude as, theextension of cylinder 16. The assembly of links 8, 10 is essentiallyrigid once the wire rope clamping forces are applied by shoes 3 and 4.

In the configuration shown, the force applied by cylinder 16 ismultiplied by approximately 1.5 times. Similarly, the travel of rope 31per stroke is 65% of the cylinder stroke. As cylinder 16 is extendedtoward the maximum, the wire rope wraps around the tail end extension 20of shoe 3 which has a constant radius relative to pin 11. This holdswire rope 31 straight between sheave 5 and shoes 3, 4 and eliminateskinking of wire rope 31, as is needed to allow the stroke distance ofthe device to be maximized.

Depending on the flow rate of the hydraulic power source and the crosssectional displacement of cylinder 16, it may take 2 to 10 seconds toextend cylinder 16. Upon reaching the end of its stroke, cylinder 16will bottom internally and cease movement. For the unit disclosedherein, the operator would then shift the aforementioned manual spoolvalve handle 37, which may be optionally mounted to mobile control stand36. The spool valve 45 is placed in the flow path between the wire ropepulling machine 33 and the hydraulic power source, not shown. Thatsource may be a custom designed power unit optimized hydraulically foruse with the puller, or it may be borrowed flow from a piece of mobileequipment such as a skid steer loader or backhoe. Extended length hoses38 and 39 terminate at the hydraulic power source.

Other hydraulic valves such as electric spool valves, valves mounted onthe mobile equipment supplying the hydraulic flow, or logic enhanceddevices can be configured to shift flow direction. Upon this shift,sequence valve 30 may again execute its task of delivering flow to thedesignated circuits in sequential order. This order would first causedelivery of fluid to port 22 of latching cylinder 1. The reversal ofload on this cylinder enables shoes 3 and 4 to release clamp load fromwire rope 31.

The sequence valve 30 will again achieve a predetermined thresholdpressure and shift flow to the second circuit in its logic scheme. Thissecond circuit is port 21 of cylinder 16. Port 19 of cylinder 1 and port15 of cylinder 16 will be connected to the return circuit throughsequence valve 30 and spool valve 45. This flow direction will causecylinder 16 to retract, pulling pin 9 toward the body of cylinder 16.With cylinder 16 swinging the system of links 10 and 8, along with shoes3 and 4, back in the direction it had been pulled from, the tension inwire rope 31 will begin to lessen; see FIG. 11. At this point, theconfiguration of the pulling device as well as the nature of the pipebursting work will determine what happens next. Specifically, there maybe substantial tension on wire rope 31 tending to pull it back the wayin the reverse of the pulling direction, and if so, the apparatus willrespond differently than if there is no such tension.

In a simplified version of the device, no means for preventing reversetravel of the cable between pulling strokes is provided. In that case,the wire rope tension is relieved totally through reverse travel of thesystem. The clamp load on the wire rope is not released from shoes 3 and4 until the elastic stretch resulting from tension in the rope is nearlyzero. In typical pipe bursting operations, especially for laterals wherethe length of the wire rope under tension is less than 100 feet, thatstretch will normally be negligible and of have no effect on theoperation. For longer pulls, such as bursting of mains, where thedistance is often over 400 feet, the stretch may be 0.5% of thetensioned cable length at some points. Additionally, elastic pipetrailing behind the bursting device may have friction induced on itsouter skin. This friction is the result of soil memory, where the soilattempts to return to it's original unexpanded condition over a periodof 1 to 5 hours. It is common for product pipe stretch to beconsiderable when bursting long distances in clay soils having a highplasticity index or swelling tendency.

In the case of longer pulls, a cable holding mechanism 60 including apivoting arm 23, a movable shoe 24 and a stationary shoe 25 should beemployed. These components restrain wire rope 31, holding it in tension,while cylinder 16 traverses the reverse portion of the stroke and lowersthe linkage for the next stroke. This mechanism 60, while not asefficient as the primary system of links 8, 10 and shoes 3, 4, serves tominimize the loss of distance due to elastic cable stretch. It may,however, allow some return of the rope to occur before the cycle of themain pulling shoes is re-initiated.

Since shoes 24, 25 are not required to pull through a distance, andtheir dependable engagement is not critical to completing a job, theirdesign can be simplified. This simplification saves cost of manufactureand more importantly, reduces overall device size of the pulling machine33. Arm 23 is mounted to the frame 48 by a pivot pin 61. Shoe 24 ispivotally mounted to the remote end of arm 23 by a pin 41 in a mannersimilar to shoe 4 and pin 14. Opposing stationary shoe 25 is rigidlybolted on pinned to side walls 28 and 29 of the frame 48.

Arm 23 serves as a stop mechanism because it causes shoes 24, 25 to moveto a wedged position when wire rope 31 reverses direction. When rope 31is moved in a productive direction (upward in the configuration shown),arm 23 swings a short distance upwardly (FIG. 10) and allows the shoe 24to swing free, merely dragging on the surface of the passing rope 31.Cylinder 16 can retract its full stroke length in order to start anotherstroke while shoes 24 and 25 hold wire rope 31 stationary.

Sheave 5 is mounted on frame 48 just below holding mechanism 60 tochange the direction of wire rope 31 from the horizontal plane in whichpipes are laid to vertical. This serves to keep the working mechanismout of the mud and sewage that will foul the excavation bottom. It is,however, optional since other means to accomplish this task could beemployed. A cable retention pin 81 is position below and inside sheave 5near the center of the position at which wire rope 31 angles upwardly.Pin 81 ensures that relaxation of wire rope 31 will not allow it to fallcompletely away from sheave 5 and become jammed to one side of it whenpulling tension is resumed.

Frame 48 includes base plate 62, end wall 59, and side walls 28 and 29which interconnect plate 62 and wall 59. Foot plate 26 supports thepulling device 33 prior to and after application of wire rope tension.It underlies base plate 62 and is wider and thicker than plate 62 toprovide secure support (see FIG. 7). Plate 26 is removably attached toplate 62 by removable connectors such as spring-loaded linch pins 63.Face plate 6 abuts the pipe being burst and is similarly removablyattachable to end plate 59 by further linch pins 63. Plate 6 serves totransfer the wire rope load from the pulling device 33 to the existingpipe or the surrounding soil. Side walls 28 and 29 provide surfaces tomount pivot pin bushings and the axle 7 for sheave 5.

Note that all systems are attached to one another in such a manner thatthe machine may be easily disassembled or assembled by pulling theassociated linch pins. This allows an operator to break the machine downinto small lightweight components and transport them into difficult toaccess locations such as basements or deep excavations. Plates 6 and 26may be positioned at the site and then connected by connectors 63 toframe 48 of the pulling device 33.

Pulling machine 33 is preferably transported using mobile control stand36 as a dolly as shown in FIG. 13. For this purpose, stand 36 has ahousing 70 with an upper end control panel 71, a frontwardly extendinglifting platform 72, a rearwardly extending handlebar 73 and a pair ofwheels 74 positioned at the lower rear corner of housing 70 so thatdevice 33 may be positioned as shown and transported by tilting housingrearwardly on wheels 74 and manually moving stand 36 and device 33 inthe position shown. Platform 72 and a front wall 76 of housing 70 may beconfigured to match the dimensions of device 33.

Set-up of the device 33 in an excavated pit 34 involves assembling thedevice over the wire rope 31 to be tensioned, or else threading the wirerope 31 through the pre-assembled device. One method is to place footplate 26 on the excavation bottom and face plate 6 on the excavationwall adjacent the existing pipeline, then bring device 33 into positionand secure plates 6, 26 as described above. In the alternative, device33 may be set into position preassembled.

Wire rope 31 is threaded through the existing pipe line 35 and throughhole 32 in face plate 6 with the machine 33 tilted on its back so thatwire rope 31 does not have to bend at sheave 5. Using this method, it isconvenient to remove cable retention pin 81 and stationary shoe 25 toprovide easier access. Pin 14 in this embodiment has a main top endportion 82 having the diameter shown in FIGS. 4–6 and a bottom endreduced diameter portion 83. Reduced diameter end portion 83 normallyprotrudes as shown in FIG. 7. When pin 14 is pulled half way out by itsT-shaped top 84, reduced diameter end portion 83 permits shoe 4 toloosen relative to shoe 3, and permits the wire rope to be manuallythreaded between shoes 3, 4. Once wire rope 31 is in position, pin 14 ispushed back in to its normal position, pin 81 is reinserted, andstationary shoe 25 is set back into position with bolts or pins. Theunit is then tipped upright to the pulling position shown in FIGS. 4–6,bending the wire rope 90 degrees around sheave 5 as shown. Uponcompletion of the job, the wire rope is removed and pulling machine 33may be partially disassembled or left fully assembled as desired.

While certain embodiments of the invention have been illustrated for thepurposes of this disclosure, numerous changes in the method andapparatus of the invention presented herein may be made by those skilledin the art, such changes being embodied within the scope and spirit ofthe present invention as defined in the appended claims.

1. A cable pulling machine, comprising: a frame; a mechanical linkageincluding first and second links having first and second cable grippingshoes disposed thereon respectively in opposed positions; a latchingcylinder actuable to cause the mechanical linkage to pivot on multipleaxes to bring the shoes into engagement with a cable disposedtherebetween; a main cylinder having one end mounted on the frame andactuable to move the linkage in a manner effective to pull the cable ina pulling stroke once the latching cylinder has brought the shoes intoengagement with the cable; and means for extending and retracting themain and latching cylinders.
 2. The machine of claim 1, wherein thefirst link is secured to the frame by a first pivot at one end portionof the first link, and has the first shoe mounted thereon at a positionoffset from the first pivot, the first shoe having a first grippingsurface configured for gripping the cable, the second link is secured toone end of the main cylinder, the second shoe having a second grippingsurface configured for gripping the cable, which second gripping surfaceopposes the first gripping surface, and the latching cylinder is mountedat opposite ends thereof to the first and second links at positionseffective for causing the first and second gripping surfaces to grip acable disposed therebetween.
 3. The machine of claim 2, furthercomprising a second pivot at which the second link is connected to anend of the main cylinder opposite the main cylinder end connected to theframe, and the second link has the second shoe mounted thereon at aposition offset from the second pivot.
 4. The machine of claim 3,wherein the latching cylinder is pivotally mounted by third and fourthpivots mounted in holes in the first and second links respectively. 5.The machine of claim 4, wherein at least one of the shoes is pivotallymounted to its associated link by a shoe pivot, on which the shoe pivotsas it tightens on the cable.
 6. The machine of claim 4, where the firstand second links overlap one another in a lateral direction transverseto the pulling stroke, and are interconnected for relative rotation by afifth pivot, the fifth pivot being located between the first and secondpivots at a position effective to cause the links to bring the shoestogether to grip the cable upon actuation of the latching cylinder. 7.The machine of claim 6, wherein the linkage is configured tomechanically magnify gripping force applied to the cable by the latchingcylinder, which latching cylinder exerts less force than the maincylinder.
 8. The machine of claim 7, further comprising a sixth pivot bywhich one end of the main cylinder is attached to the frame.
 9. Themachine of claim 8, further comprising a control unit including: acircuit valve which, upon actuation for a pulling stroke, activates thelatching cylinder until a predetermined pressure is reached, and thenactivates the main cylinder; and a manual control for cycling themachine to perform a pulling stroke and then reset to a startingposition to repeat for a successive pulling stroke.
 10. The machine ofclaim 9, further comprising means for holding the cable in tensionbetween pulling strokes.
 11. The machine of claim 10, wherein the frameis generally L-shaped including a side wall and a bottom wall which actas reaction surfaces for the cable pulling machine, the first pivot isattached by a first bracket to the side wall of the frame, and the sixthpivot is attached by a bracket to the bottom wall of the frame.
 12. Themachine of claim 1, wherein at least one of the shoes is pivotallymounted to its associated link by a shoe pivot, on which the shoe pivotsas it tightens on the cable.
 13. The machine of claim 1, furthercomprising a control unit including: a circuit valve which, uponactuation for a pulling stroke, activates the latching cylinder until apredetermined pressure is reached, and then activates the main cylinder;and a manual control for cycling the machine to perform a pulling strokeand then reset to a starting position to repeat for a successive pullingstroke.
 14. The machine of claim 1, wherein the linkage mechanicallymagnifies forces applied to the cable by the latching cylinder, whichlatching cylinder is smaller than the main cylinder and exerts lessforce than the main cylinder.
 15. The machine of claim 1, furthercomprising means for holding the cable in tension between pullingstrokes.
 16. The machine of claim 1, wherein one of the cable grippingshoes has a curved extension, and the linkage pivots to wrap the cableabout the extension at the end of a pulling stroke.
 17. The machine ofclaim 1, wherein the frame is generally L-shaped including a side walland a bottom wall which act as reaction surfaces for the cable pullingmachine.
 18. The machine of claim 17, wherein the side wall of the framehas a cable opening therein, and a sheave rotatably mounted on the frameproximate the cable opening whereby a cable extending through the cableopening is wound around the sheave as it extends from the sheave to thecable gripping shoes.
 19. The machine of claim 18, further comprising anarm pivotally mounted on the frame intermediate the sheave and the cablegripping shoes, the arm having means on a distal end thereof forarresting return movement of the cable between pulling strokes.
 20. Apipe bursting system, comprising: a cable pulling machine including aframe, a mechanical linkage including first and second links havingfirst and second cable gripping shoes disposed thereon respectively inopposed positions, a latching cylinder actuable to cause the mechanicallinkage to pivot on multiple axes to bring the shoes into engagementwith a cable disposed therebetween, a main cylinder having one endmounted on the frame and actuable to move the linkage in a mannereffective to pull the cable in a pulling stroke once the latchingcylinder has brought the shoes into engagement with the cable, and meansfor extending and retracting the main and latching cylinders, whereinlines of action for each link move from an initial substantiallyparallel position to a non-parallel position effective to magnifyclamping force exerting by the gripping shoes; a cable configured to bepulled by the cable pulling machine; and a mole attachable to one end ofthe cable, the mole configured for bursting an underground pipeline. 21.A cable pulling machine for pulling a pipe bursting mole through anunderground pipeline, comprising: a frame; a mechanical linkageincluding a pair of first and second cable gripping shoes disposed inopposed positions, first and second links having the first and secondcable gripping shoes mounted thereon, a first pivot by which the firstgripping shoe is pivotally mounted to the first link and a second pivoton which the first and second links are mounted for relative rotation; alatching cylinder actuable to cause the mechanical linkage to bring theshoes into engagement with a cable disposed therebetween; and a maincylinder having one end mounted on the frame and actuable to move themechanical linkage in a manner effective to pull the cable in a pullingstroke once the latching cylinder has brought the shoes into engagementwith the cable, which movement of the main cylinder during the pullingstroke generates additional clamp load by the first and second cablegripping shoes on the cable.
 22. The machine of claim 21, wherein themechanical linkage further comprises a third pivot by which one end ofthe latching cylinder is pivotally mounted to the first link.
 23. Themachine of claim 21, further comprising means for extending andretracting the main and latching cylinders.